Fibril‐Type Textile Electrodes Enabling Extremely High Areal Capacity through Pseudocapacitive Electroplating onto Chalcogenide Nanoparticle‐Encapsulated Fibrils

Abstract Effective incorporation of conductive and energy storage materials into 3D porous textiles plays a pivotal role in developing and designing high‐performance energy storage devices. Here, a fibril‐type textile pseudocapacitor electrode with outstanding capacity, good rate capability, and excellent mechanical stability through controlled interfacial interaction‐induced electroplating is reported. First, tetraoctylammonium bromide‐stabilized copper sulfide nanoparticles (TOABr‐CuS NPs) are uniformly assembled onto cotton textiles. This approach converts insulating textiles to conductive textiles preserving their intrinsically porous structure with an extremely large surface area. For the preparation of textile current collector with bulk metal‐like electrical conductivity, Ni is additionally electroplated onto the CuS NP‐assembled textiles (i.e., Ni‐EPT). Furthermore, a pseudocapacitive NiCo‐layered double hydroxide (LDH) layer is subsequently electroplated onto Ni‐EPT for the cathode. The formed NiCo‐LDH electroplated textiles (i.e., NiCo‐EPT) exhibit a high areal capacitance of 12.2 F cm−2 (at 10 mA cm−2), good rate performance, and excellent cycling stability. Particularly, the areal capacity of NiCo‐EPT can be further increased through their subsequent stacking. The 3‐stack NiCo‐EPT delivers an unprecedentedly high areal capacitance of 28.8 F cm−2 (at 30 mA cm−2), which outperforms those of textile‐based pseudocapacitor electrodes reported to date.

Subsequently, 12 mmol Na2S2O3 was added and stirred for 15 min which made the mixture solution to be clear. Then, 0.9 M NaBH4, a reducing agent, in deionized water (10 mL) was poured in the solution. After 30 min, the deep brown toluene phase was separated from the mixture and washed several times with 10 mM HCl, 10 mM NH4OH, and deionized water.
LbL assembly of (TOABr-CuS NP/Cys)n multilayers: Substrates including Si wafers, Ausputtered Si wafers, quartz glasses, and QCM electrodes were irradiated with a UV ozone cleaner for 30 min. Above mentioned substrates and bare cotton textiles (0.5 x 3 cm 2 ) were first dipped into Cys solution (in ethanol, 2 mg mL -1 ) for 30 min to form robust underlayer, then washed twice with pure ethanol to remove the weakly adsorbed Cys molecules. The formed Cys-coated substrates were dipped into TOABr-CuS NPs solution (in toluene, 30 mg mL -1 ) for 30 min, then washed with pure toluene to eliminate the weakly adsorbed NPs. In the following process, TOABr-CuS NP coated substrates were dipped into Cys solution for 5 min to replace bulky TOABr ligands to small Cys ligands. After this ligand exchange reaction-induced adsorption process, weakly adsorbed Cys layer was re-washed out using pure ethanol. These processes were repeated until the desired bilayer number (n) of (TOABr-CuS NP/Cys)n multilayers was reached.
Preparation of Ni-EPT: Electroplating solution (Watts bath) was prepared using 240 g L -1 NiSO4·6H2O, 45 g L -1 NiCl2·6H2O, and 30 g L -1 H3BO3 with deionized water. [S1] (TOABr-CuS NP/Cys)5-coated textiles (as a cathode) and nickel plate (as an anode) were immersed in the electroplating solution. Electroplating process was performed by adjusting the external current and electroplating time. The formed Ni-EPT was washed several times with deionized water, and then dried in a vacuum oven.
Preparation of Ni-CRT: Electroless deposition, using chemical reduction of Ni precursors on textile, was prepared from following process. 0.5 x 3 cm 2 sized cotton textile was first immersed in a sensitizing solution containing 50 mM SnCl2·2H2O and 0.15 M HCl (37 %) for 10 min. After this dipping process, the textile sample was dipped into an activating solution containing 0.6 mM PdCl2 and 0.03 M HCl. Finally, the sample was immersed in electroless plating bath held at 80 ℃, consisting of 240 g L -1 NaH2PO2·H2O, 45 g L -1 NiSO4·6H2O, 50 g L -1 NH4Cl, 30 g L -1 Na3C6H5O7·2H2O, and adjusted pH to 9 with 0.27 % NH4OH solution.
After stirring for 120 min, sample was washed with deionized water and dried in a vacuum oven.
Preparation of NiCo-t: NiCo-LDH layer was additionally electroplated in a traditional 3-electrode electrochemical cell which is composed with Ni-EPT, Pt coil, and Ag/AgCl (3 M NaCl) as a working, counter, and reference electrode, respectively. Electroplating was carried out in a growth solution (10 mM Ni(NO3)2·6H2O and 5 mM Co(NO3)2·6H2O in deionized water) at a constant current density of 3 mA cm -2 for given electroplating time. After the process, the electrode was washed with deionized water, and then dried in a vacuum oven. In the case of performing the electroplating times for 15, 30 and 45 min, the formed electrodes were designated as NiCo-15, NiCo-30, and NiCo-45, respectively. Particularly, NiCo-60 is defined as NiCo-EPT because of the use of the same experimental condition (i.e., electroplating time ~60 min). In this case, the thickness of the NiCo-EPT was increased up to approximately 1.0 mm after electroplating process. For the stacked NiCo-EPTs, the upper part of each NiCo-EPT was attatched with silver paste until the desired stacking number was reached. After this process, the stacked NiCo-EPTs were fixed with epoxy.
Preparation of NiCo/Ni foam: Commercial Ni foam were ultrasonically cleaned in acetone, ethoanol, and deionized water for 30 min to remove organic residues on the surface, and then dried in vacuum oven for 24 h. After these cleaning processes, NiCo-LDH layer was electroplated onto the Ni foam using the the same electroplating conditions (3 mA cm -2 for 60 min) performed for the preparation of NiCo-EPT.
Preparation of L-NiCo-EPT: Ni was electroplated on (TOABr-CuS NP/Cys)5-coated textiles using the external current density of 50 mA cm -2 for 10 min, which resultantly exhibited the relatively high sheet resistance of 20 Ω sq −1 . Subsequently, NiCo-LDH layer was electroplated under the same condition as the preparation of NiCo-EPT (i.e., 3 mA cm -2 for 60 min).

Preparation of CT:
Bare cotton textiles was heated at 100 ℃ for 30 min to remove residual moisture. In the following process, heating temperature was increased up to 950 ℃ at a rate of 3 ℃ min -1 , and then held for 3 hr. These processes were carried out in the furnace under a nitrogen atmosphere. After cooling to ambient temperature, the carbonized textile (CT) was taken out from the furnace. In this case, the formed CT exhibited a sheet resistance of 9  sq -1 .
Preparation of the AFPs: NiCo-EPT and CT were composed of cathode and anode for AFPs, respectively. Area (or mass) ratio of cathode and anode was adjusted based on the following charge balance equation: [S2] = × ∆ × (1) where , , ∆ , and refers to stored charge, areal or specific capacitance (F cm -2 or F g -1 ), potential window (V) during the CV or GCD operation, and area (cm 2 ) or mass (g) of the active material, respectively. Based on the equation, areal ratio of the NiCo-EPT and CT was calculated to be 2.54 : 1.
Where 0 indicates the resonant frequency of fundamental mode of the crystal (5 MHz). A, , and refers to electrode surface area (cm 2 ), density (2.65 g cm -3 ), and shear modulus (2.95 x 10 11 g cm -1 s -2 ) of QCM electrode respectively. By considering each values, above equation can be simplified as equation (4): FTIR analysis of multilayers on Au-sputtered Si wafers was conducted using a Cary 600 (Agilent Technologies) operated in Advanced Grazing Angle (AGA) mode with at a 4 cm -1 resolution, and obtained data were plotted using spectrum analysis software (OMNIC, Thermo Fisher Scientific). X-ray diffraction (XRD) patterns were obtained using a SmartLab instrument (Rigaku) with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) analysis were performed by X-TOOL (ULVAC-PHI) with monochromatic Al Kα radiation as the excitation source. The sheet resistance was measured using four-probe measurement method with Loresta-GP MCP-T610 (Mitsubishi Chemical Analytech).
Electrochemical measurements: All the electrochemical measurements were carried out with an Ivium-n-Stat electrochemical workstation (Ivium Technologies). Electrochemical performances of all single electrodes were evaluated in a three-electrode cell system and asymmetric supercapacitor (ASCs) was evaluated in two-electrode cell system. In the threeelectrode configuration, all single electrodes (active area ~0.5 x 1.5 cm 2 ), Pt mesh, and Hg/HgO (1M NaOH) were used as the working, counter, reference electrode, respectively. 6 M KOH was used as an electrolyte for three and two-electrode cell system. EIS measurements were performed in the frequency range of 10 5 to 0.1 Hz with a perturbation amplitude of 0.01 V.
The capacitance (C) values of the electrodes were calculated from the GCD discharge profiles according to the following equation (5): [S4] = ∆ ∆ (5) Where , ∆ , and ∆ refers to applied current (A), discharging time (s), and operating potential window (V), respectively. The variable indicates the area (cm 2 ), mass (g), or volume (cm 3 ) of the active material.
Warburg impedance coefficient ( ) was obtained from the slope values in the following equation (6): [S5] Where indicate angular frequency. Figure S1. Photographic images of TOABr-CuS NPs synthetic process, which showed stable dispersion in toluene without particle aggregations.       showed the presence of α, β-Ni(OH)2, which attributed from partial surface oxidation.